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Biasing Cache Threshold Pointers Toward Less Pre-Staging

Publishing Venue

IBM

Related People

Oliver, JK: AUTHOR

Abstract

A technique is described whereby computer cache storage performance is improved by biasing cache threshold pointers toward less pre-saging. The concept adjusts the threshold pointers so that when the hit ratio varies little, or not at all, over the entire range of threshold pointers, a smaller, average effective block length is used. This results in less pre-staging and, therefore, increases the performance of cache operations. In response to the need for greater performance many disk attachments implement cache algorithms to try to maintain the most recently or most often requested data directly in the cache memory. When the operating system requests data, many times the request can be met entirely with data contained in the cache, thereby reducing access time.

Country

United States

Language

English (United States)

This text was extracted from a PDF file.

This is the abbreviated version, containing approximately
24% of the total text.

Page 1 of 5

Biasing Cache Threshold Pointers Toward Less Pre-Staging

A technique is described whereby computer cache storage
performance is improved by biasing cache threshold pointers toward
less pre-saging. The concept adjusts the threshold pointers so that
when the hit ratio varies little, or not at all, over the entire
range of threshold pointers, a smaller, average effective block
length is used. This results in less pre-staging and, therefore,
increases the performance of cache operations. In response to the
need for greater performance many disk attachments implement cache
algorithms to try to maintain the most recently or most often
requested data directly in the cache memory. When the operating
system requests data, many times the request can be met entirely with
data contained in the cache, thereby reducing access time. Techniques
have been implemented in the past which improve the alignment of
cache blocks with a short sequence pattern of disk references, called
pre-staging. Here, one or more extra blocks of data are entered into
cache when a read miss occurs. This pre-staging is performed in the
hopes that the user is reading sequentially and will request data
from those blocks in the near future. Essentially, the pre-staging
of the data is in anticipation of the possibility of saving time in
the future. The read requests would then come directly from cache,
resulting in a gain in performance. However, occasionally the user
is not reading sequentially and, therefore, does not request data
from the pre-staged blocks. In this case, the extra time required to
read the pre-staged data from the disk is a detriment to the
performance. This prior technique also explained how different
values of cache threshold pointers resulted in different average
effective block lengths. Another technique concentrated on the
automatic adjustment of disk cache operation based on tracking the
read hit ratios. Here, a means was provided whereby the cache
threshold pointers were adjusted based on the read hit ratio. The
threshold pointers were periodically adjusted and the cache hit ratio
was measured at each value of the threshold pointers. In this
manner, the threshold pointers were adjusted to keep the hit ratio at
a maximum. However, a problem existed in this concept in situations
where the hit ratio did not vary significantly over the entire range
of values of threshold pointers. When the average effective block
length was increased by varying the threshold pointers, the time
required to read the pre-staged data from the disk increased. If the
hit ratio does not rise sufficiently so as to compensate for this
extra time, the operational performance decreased. The concept
described herein attempts to overcome the various deficiencies by
concentrating on the optimization of cache performance. This is done
by biasing the threshold pointers toward less pre-staging.

Algorithms are used to take into account the performance gain or
loss, caused by the change...